1. Technical Field
The invention relates to gear metering pumps and in particular to gear metering pumps for moving a predetermined amount of a compounded elastomeric material from the output of an extruder into a forming die. More particularly, the invention relates to such a gear metering pump which enables rapid cleanout of the interior chamber and meshing gears and lubrication of the gear shafts in a simpler, compact and lighter weight construction.
2. Background Information
Gear pumps are well known and widely used in the plastics and fiber industry. They are generally made by encasing a set of meshing gears within a close fitting casing or housing which is formed with feed and discharge ports. The material being transferred, fills the spaces between the gear teeth and as the gears turn, a volume element of the fluid material is trapped between flanks of adjacent gear teeth by close tolerances of housing envelope vs. tooth crests and ends. This volume element is positively transported to the discharge outlet of the chamber where it is displaced by meshing of opposing teeth. Thus, gear pumps are identified as positive displacement devices.
Gear pumps are generally classified by the services provided, such as metering, pressure boosting and volume transfer, as well as by the materials handled, such as polymers or low viscosity or inviscid liquids. Metering and pressure pumps are precision machines with exceedingly small internal clearances. Polymer metering pumps were developed primarily for synthetic fiber extrusion. Transfer pumps tend to have larger clearances for low friction at high RPM and allows fluid to slip or leak back to the feed port if the outlet is plugged. Certain of these prior art gear pumps include recycling relief valves to prevent excess pressure
Most known gear pumps are lubricated by the fluid being pumped or transferred therethrough. The need to positively force polymeric fluids through the bearings has been met by several various designs present today in the gear metering pump industry. Some of these prior art metering pumps have gear diameters two or more times the shaft diameters such that a pumping action is created by the twisting action of the gear end face vs. the housing sidewall which provides an effect referred to in the polymer industry as a necktie effect. Lubricant flow depends more on clearance, pump speed and polymer rheology than it does on pressures and is self-stabilizing. Pumps usually are assembled with slightly insufficient clearance so initial sidewall wear is rapid. However, increased clearance at the gear faces, pumps more lubricant so wear rate decreases. Even so metering pumps are usually removed for cleaning and maintenance every few weeks. Most transfer pumps depend more on internal pressures to force lubricating polymer into the bearings. Flow through the bearings may be assisted by spiral grooving of the shaft or sleeves.
Early gear pumps allowed a small lubricant stream to pass through the bearings and leak outside the pump. This was not only undesirable but became unacceptable with the advent of dangerous fiber spinning solvents and fluids that could seize the pump shaft when they dried or froze. Absolute shaft seals were then developed and pumps were internally ported to return the bearing lubricant to the feed stream.
Gear pumps designed for polymers, especially gear metering pumps, are difficult to disassemble for cleaning and maintenance because parts must be designed for high strength as well as very tight fit, and usually are held together by the stiff polymer being transferred thereby. Maintenance on these polymer gear metering pumps usually requires various wrenches, mallets, pry-bars, gear puller and arbor presses. Very often a separate or spare pump is required to be maintained to restore production while the other gear pump is taken from the production line for maintenance.
A unique problem encountered in extruding profiles which will then be assembled into tires, is the myriad of different elastomeric compounds which must be used. To avoid contamination of one compound with another, extrusion lines may be cleaned twenty or more times per day.
The present invention relates to the use of gear pumps for a compounded elastomeric material. Such material, which hereinafter may be referred to as the compound, includes high-molecular weight elastomeric polymers which makes the compound very viscous and somewhat elastic, thereby creating extreme extrusion pressures and stresses. Such compound will contain solid reinforcing particles, such as carbon and/or silica, which makes it abrasive and which increases its viscosity. It also will contain curatives which make it thermosetting (vulcanizing) and scorch sensitive. Also upon cessation of shear, the compound assumes the properties of a solid. These properties make such a compound difficult for use with gear metering pumps.
The recycling of the material being passed through the gear metering pump used as the lubricant, is unsatisfactory where the material is such a compounded elastomeric material, due to the above described properties and particularly due to the risk of the compound vulcanizing and plugging the channels and ports through which the lubricating streams are transferred back to the main feed stream of material. Likewise, lubrication is uncertain in startup, especially when the extrusion head is empty or the amount of initial lubricant is insufficient for proper lubrication of the bearings. Also a problem exists where the material is not recycled but is discharged through torturous passages which easily become plugged.
Another problem present in gear metering pumps, especially for metering of compounded elastomeric material, is that the use of a positive seal for the power driven shaft of the meshing gear, as it exits the gear pump housing, creates a substantial heat problem. The creation of such heat is not a problem in extruding most fibers and films but is critical when the material is such a compounded elastomeric material. Control of the temperature of the compound is critical to ensure it is at a high enough temperature for extrusion through the adjacent die, yet not too high to cause partial vulcanization. It has been found that shaft seals can provide as much as 50% of the heat, which is added to the compound being metered by the meshing gears of the pump, thus adding to the heat control problem.
Therefore, the need exists for a gear metering pump for use in metering a compounded elastomeric material, which eliminates the generation of heat by the use of positive shaft seals, which enables access to be gained easily to the gear chamber and meshing gears for cleanout, and which enables the compound to be used as the lubricant without the problem of plugging of the lubricant flow channels or ports.